Electro-absorption modulators are used to modulate light in optical telecommunications applications. Typically, an electro-absorption modulator modulates light generated by a continuous light source. The electro-absorption modulator typically modulates light by either allowing or preventing light from passing through the electro-absorption modulator. One of the main parameters that characterize the light modulation performance of an electro-absorption modulator is the extinction ratio. The extinction ratio is the ratio of the maximum power output to the minimum power output of the electro-absorption modulator. A higher extinction ratio is typically the result of a higher absorption of light through the creation of more electron-hole pairs in the active layer.
Electro-absorption modulators capable of operating at data rates on the order of 40 Gb/s are of interest for optical telecommunications applications. Electro-absorption modulators are typically based on the quantum-confined Stark effect. Applying an electric field across the quantum well structure changes the effective band gap energy of the quantum well structure through the quantum-confined Stark effect. Electro-absorption modulators absorb light when a reverse bias is applied to the p-i-n junction. Because little current flows when the reverse bias is applied, the modulation speed is limited by the time required to charge and discharge the capacitance of the electro-absorption modulator.
There are a number of tradeoffs associated with multiple quantum well design of electro-absorption modulators and the impact on performance parameters. Overall electro-absorption modulator design and operation typically represents a tradeoff among limitations. A higher extinction ratio may be achieved by increasing absorption through longer modulators, more quantum wells or higher voltage swing operation. However, the modulation rate is adversely effected because longer modulators result in higher capacitance and increasing the number of quantum wells increases carrier extraction time.
As noted, typical electro-absorption modulators are operated under reverse bias which results in an applied electric field that causes a separation in the electron and hole wavefunctions where the hole distribution is distributed toward the p-doped side and the electron distribution is distributed toward the n-doped side of the quantum well. This physical separation between photogenerated carriers translates into reduced absorption which reduces the extinction ratio compared to that obtained if overlap between hole and electron wavefunction is maintained. FIG. 1 shows this effect by examining the photocurrent absorption spectra at room temperature for an eight quantum well InGaAsP modulator. Curve 101 shows a nearly ideal photocurrent absorption spectrum at zero reverse bias, a sharp bandedge transition at λ˜1490 nm, along with an excitonic absorption resonance. As the reverse bias is increased to about 1.25 volts as shown by curve 105, to about 2.5 volts as shown by curve 110 and to about 3.75 volts as shown by curve 115, the absorption edge shifts to longer wavelengths because of the quantum-confined Stark effect. The absorption decreases in magnitude as the reverse bias is increased due to the increasing separation between the hole and electron distribution in the quantum well regions.